The episode begins at the Gotham university with Firefly and his new love interest Blaise stealing a radioactive isotope. However, while they're pulling the heist off, a group of cops bust in and confront them. Firefly threatens to drop the isotope, which would kill them all. This causes the cops to back off and allows Firefly and Blaise to make their escape. Later at an apartment, Firefly begins to use the isotope he stole to modify his jetpack while expressing his love for Blaise. Blaise explains the improvements the isotope will bring to Firefly's battlesuit. Firefly then reveals his plan to get steal lots of money from Gotham, flee, and start a nice life. Blaise turns on the TV, revealing a report on their recent crime in which the reporter refers to Firefly as "Bee Man". Firefly gets upset that no one gets his name right and doesn't notice the extreme amount of heat he's applying to the isotope. This results in the isotope exploding, blowing up the apartment.

After the explosion, the apartment is in ruins. Blaise is fine. However, Firefly has a strange orange glow around him. Batman, Robin, and Batgirl learn about the incident and head to the sight of the explosion. In the destroyed apartment, Batman, Robin, and Batgirl find the remains of Firefly's jetpack. Robin finds a picture of Firefly with Blaise and Batman reveals that Blaise turned to crime when her project at Gotham university was shut down. Batman then notices the equations written down on the chalkboard and expresses some concern. Batgirl also notices that the isotope that blew up contained much more energy than what was being displayed on her scanner and questions this. Meanwhile, Blaise attempts to get Firefly to a hospital. Firefly refuses, as it would result in them going to jail. Blaise notices it's getting hotter, but Firefly claims it's just the heat of the car and gets out. A car almost runs over Firefly, resulting in him pounding on the car. The car starts to smoke and Firefly notices the orange glow around his hands. A cop car comes after Firefly, but he uses his new powers to light it on fire. Firefly, now aware of his ability to generate fire destroys an ATM machine. Unfortunately, when Firefly tries to take the money, it gets burnt to ashes. He simply shrugs at that he might control it later. Batman, Robin, and Batgirl come after Firefly. Batman dodges Firefly's fireballs while Batgirl prevents Blaise from getting away. Robin uses a fire hose against Firefly which seems effect at first, but then Firefly generates even stronger heat than the water from the hose. Batman throws some ice bombs that freeze Firefly and it finally seems like he's stopped for good. However, Firefly's eyes turn orange and his skin gets darker. An explosion occurs and once it clears, Firefly emerges with black skin and red lines all over his body. Firefly declares that from know on, he will be known as Phosphorus.

After attempting to incinerate Batman, Robin, and Batgirl, Phosphorus hijacks a truck with Blaise. Phosphorus discovers that with his transformation, he has become too hot to sit in the front seat of the truck so he sits in the back, but even there he's too hot for. Blaise begins to question if she wants to continue working with Phosphorus but he convinces her to stay with him. Phosphorus breaks into a bank, takes out the guards with fireballs, and proceeds to steal to gold, only to find out he's so hot, the gold melts when he touches it. Batman, Robin, and Batgirl arrive and fight Phosphorus. Batman fires a small device that encases Phosphorus in ice. However, not even this is enough to stop Phosphorus. Phosphorus defeats Batman, Robin, and Batgirl, but they manage to escape. Back at the batcave, the computer reveals that in time Phosphorus will get hot enough to melt into the ground, taking Gotham with him. Batgirl goes to look for Phosphorus.

At a grocery store, Phosphorus attempts to eat raw meat, but even that is burnt to ashes within seconds of him touching it. Blaise also notices Phosphorus starting to sink into the ground. After realizing he can no longer ride, eat, or even stand, Phosphorus goes insane and starts launching fire everywhere. Blaise runs into Batgirl, but Phosphorus spots them and thinks Blaise has turned against him. Enraged, Phosphorus heads towards the power plant.

Batman suits up in a radiation shielded suits and prepares to take on Phosphorus. Phosphorus enters the power plant but is stopped by Batman. Robin uses a rope to tie Phosphorus up but he burns it to ash. Batman's new suit proves effective against Phosphorus, but it doesn't protect him when Phosphorus unleashes a blast of stronger heat. Phosphorus goes deeper into the power plant and is stopped by Batgirl and Blaise. Phosphorus ignores them and begins to apply heat to the reactor. Batman throws a fire extinguisher at Phosphorus which explodes when he touches it. With the explosion catching Phosphorus off guard, Batman opens the reactor, releasing liquid nitrogen which covers Phosphorus, finally defeating him.

Phosphorus is taken to Arkham and a doctor reveals that while they are able to use medicine to shut down his powers, the physical and mental damage done to Phosphorus is permanent. Blaise reveals that she's done with Phosphorus and is taken into custody. The episode ends with the doctor telling a grief stricken Phosphorus that it's late and to get some sleep.